ABSTRACT/PROJECT SUMMARY Kaposi sarcoma (KS) is among the most common HIV-associated malignancies worldwide, and a leading cause of cancer morbidity and mortality in sub-Saharan Africa. A growing body of evidence demonstrates the tremendous ability of T-cells to mediate tumor regression, and several characteristics of tumor-infiltrating lymphocytes (TIL) are associated with superior survival in a range of solid tumors. While KS development is strongly associated with immune dysfunction in the context of HIV infection, little is known about immune responses to KS, and no studies have evaluated features of TIL in KS tumors in relation to clinical outcomes. In response to PQ5, we propose the first study to define the characteristics of effective T-cell responses to KS in adults with and without HIV infection. Preliminary studies by our group demonstrate that T-cells commonly infiltrate HIV+KS tumors, and that higher T-cell receptor (TCR) clonality in tumor-infiltrating lymphocytes (TIL) is associated with improved treatment response. We have further observed the expansion of specific T-cell clones in post-treatment TIL in patients who ultimately achieve a complete response to therapy, and we hypothesize that these clones are reactive with KS tumor cells and contribute to tumor regression. Importantly, we have also found high expression in KS tumors of several immune exhaustion markers, including PD-1,CTLA-4, and LAG-3, which may be increased in HIV-infected individuals, and contribute to their inability to mount effective KS immune responses. By comparing TIL in HIV+ and HIV-KS tumors, we hope to learn about the impact of HIV on T-cell responses. We further postulate that persons with HIV-KS, who have no known immunodeficiency and typically enjoy superior treatment outcomes, will generate more effective antitumor responses that can guide therapeutic strategies to enhance similar T-cell responses in HIV+KS. We will test our hypotheses by utilizing the extensive KS tumor repository from our ongoing study of adults with HIV+KS and HIV-KS receiving treatment at the Uganda Cancer Institute through the following aims: 1) To define the spatiotemporal characteristics of TIL in KS tumors, and to determine if features of intratumoral CD4+ and CD8+ T-cell infiltration are associated with response to therapy and with 1-year survival. 2) To evaluate the diversity and clonal composition of the T-cell receptor ? chain (TRB) repertoire of TIL in KS tumors, and to determine if clonal composition and/or specific clonal populations in TIL are correlated with response to therapy and 1-year survival. 3) To define the expression of viral and immune-related host genes in KS tumors, and to identify features associated with response to therapy and 1-year survival. Through an improved understanding of adaptive immune responses to KS, these studies will contribute to the development of targeted and effective immune-based staging and treatment strategies, including immune checkpoint inhibition and therapeutic vaccination, to improve response and survival in persons with HIV+KS.